A printed circuit board laminate generally comprises several sheets of epoxy impregnated fiberglass cloth, called "pre-preg", sandwiched on each side by a sheet of copper foil. A process for manufacturing a copper clad laminate, as generally known in the art, commences with coating a web of glass cloth with a solution of epoxy resin to form a B-stage. The B-stage is sheeted to a suitable size, and a predetermined number of sheets are piled up to achieve a predetermined thickness of the final pre-preg laminate. Copper foil of suitable sheet size is then placed on each side of the B-stage pre-preg. The copper B-stage pre-preg copper laminate assembly is then placed between a pair of stainless steel press plates and inserted into a known vacuum hot press for pressing of the laminate. In the hot press, the assembly is heated up which causes the epoxy resin in the B-stage to melt and harden, forming the insulating core of the laminate. Subsequently, the laminate is cooled and removed with the press plates from the hot press. The laminate is then separated from the press plates which results in a final product while the press plates are reused for the next cycle of manufacturing laminates.
The intended use of the laminates requires etching of conductive lines so as to effect electrical connections on the laminate surface. These lines are typically outlined and etched with photochemical means and may be as narrow as 0.002-0.005 inches. Consequently, any small surface defect on the copper surface may interface with the formation of these circuit lines and is therefore undesirable.
In known large scale laminate manufacturing plants, a multiple assembly of press plates, copper foils and B-stage pre-pregs have to be prepared for insertion into the press. The multiple assembly, known in the industry as a "book", may be typically contain eight to twelve laminates. In order, a "book" may comprise a carrier plate on which is stacked, in order, press pads, approximately twelve press plates each containing one laminate, one additional press plate, and press pads. The "carrier plate" is generally a hardened steel plate which carries the book of laminates from the laminate assembly point to the press area, then through breakdown of the book after pressing. The carrier plate is used to absorb the abuse caused during the material handling of the book. The thickness of the carrier plate is generally about 0.118 inches. The length and width of the carrier plate is approximately six inches larger in each direction than the largest press plate. In addition, T handles or tangs may be attached to each transverse end of the carrier for pulling the books on and off loading and unloading elevators.
Each press plate is generally a stainless steel plate on which the laminate is assembled. The press plate length and width is generally the same as the pre-preg length and width. The copper foil length and width dimensions are generally one inch larger in each direction than the press plate.
A great deal of effort has been expended to create layup systems which would assure a minimized contamination of the copper foil and stainless steel plate surfaces to obtain non-contaminated laminate surfaces. In one known laminate layup system, the book of laminates is started by placing the first stainless steel press plate onto a carrier plate/press pad base. The first or bottom copper foil is placed on the press plate, followed B-stage and the second or top copper foil. Then the process of placing the next press plate, bottom copper foil, B-stage, and top copper foil is repeated until the desired number of assemblies for the book is attained. The book is then removed for pressing and assembly of a new book starts.
The process of building the book is typically performed in a "clean room", i.e., air entering the room is filtered. A clean room is preferred to minimize entry of particulate contaminants. Accordingly, the press plates are cleaned by mechanical means to minimize such contaminants. The largest problem for achieving defect-free surfaces comes from handling B-stage material in conjunction with the copper foil and stainless steel plates. This is because the B-stage contains very brittle epoxy resin and the resin particles and residues of glass fibers from pre-preg cutting and sheet surface abrasion is highly static and numerous. When handled in one room with clean surfaces of press plates and copper foils, either manually or with overhead handlers, some of these particles inevitably contact onto the critical surfaces and create surface defects. Such defects often require the laminate to be scrapped since various criteria for determining surface defects are stringent. For example, a dent of 0.015 inches in diameter appearing in the outer skin of the copper foil (top or bottom) would disqualify a laminate sheet as per military specification MIL-P-13949-G. Much more stringent requirements are also in place within the industry.
In a known modification to the standard layup system described above, a book of laminates reciprocates between two rooms. In one room, press plates and copper foils are handled and, in the second room, pre-preg is placed onto the copper foil underneath. This system tends to improve avoidance of contamination somewhat. However, the exposed surface of the B-stage has to enter the "metal" clean room and decelerate to a stop which creates slight air turbulence with further turbulence created when placing the top copper foil onto the exposed pre-preg. In addition, the movement of the buildup from room to room is slow and the large surface of the book and carriage tends to carry particulate contamination from room to room.
In a prior layup operation, used by the assignee of the present invention, the copper foils, B-stage pre-preg, press plates and carrier trays or plates are all located in the same room where the component assembly or layup is performed. Generally, a layup station has a central work surface such as a lift table or conveyor table that serves as the assembly point for the laminate components. Prior to commencement of the layup operation, a carrier plate is placed into position on the layup station's central work surface. Depending on the operation, the carrier plate may be placed manually or with automated conveyor equipment. The foils, B-stage and clean press plates are located around the work station so that they may be placed onto the carrier tray at appropriate intervals. With a fully manual system, stacks of properly sized foil, B-stage, and clean press plates are arranged around the carrier tray station so that minimum operator movement is required during the layup operation. When a greater degree of automation is used, foil and press plates are not stored around the layup station in stacks but are brought in a continuous manner with overhead equipment as needed. Typically, however, the B-stage pre-preg is stacked near the central layup table.
Prior to starting the layup operation, several sheets of kraft paper are placed on the carrier plate. The kraft paper serves as a thermal lag material and also improves pressure distribution characteristics during lamination.
After the carrier tray and kraft paper are in position, the layup process can begin. The sequence of operations, whether performed manually or by automated equipment, is as follows:
1. A clean, polished press plate is placed onto the kraft paper that is located on the carrier tray. PA0 2. A sheet of copper foil is placed onto the press plate, with the shiny side facing down, and the adhesion enhancement side facing up. PA0 3. B-stage, one sheet or many, is placed onto the copper foil. Care is taken to ensure alignment. PA0 4. A sheet of copper is placed onto the B-stage, with the shiny side facing up, and the adhesion enhancement side facing toward the B-stage. PA0 5. Another press plate is placed on top of the foil. PA0 1. A clean, polished press plate is placed onto the kraft paper that is located on the carrier tray. PA0 2. A sheet of copper foil is placed onto the press plate, with the shiny side facing down, and the adhesion enhancement side facing up. PA0 3. The carrier tray is shuttled through the opening in the wall into the B-stage room. PA0 4. B-stage, one sheet or many, is placed onto the copper foil. Care is taken to ensure proper alignment. PA0 5. The carrier tray is shuttled back into the copper/plate room (with previously placed components). PA0 6. A sheet of copper foil is placed onto the B-stage, with the shiny side facing up, and the adhesion enhancement side facing down. PA0 7. Another press plate is placed on top of the foil.
Steps two through five are repeated until eight to twelve laminates have been stacked on top of the carrier tray. Sheets of kraft paper are placed on top of the last press plate. This then completes a "book", which is transported to a load elevator to await pressing, or lowered on a lift table to allow another carrier tray to be placed on top of it (most manual systems lay up an entire press load, 10-20 books, on a single lift table, and then transport the entire load to the press).
In an alternative layup operation, the copper foil, B-stage, press plates, and carrier plates used in the shuttle layup system are not located in the same room as in the above-mentioned standard layup operation. Instead, the copper foil and press plates are located in one room while the B-stage is placed in an adjacent room. Stacks of properly sized foil, clean press plates, and B-stages are placed ergonomically in their respective rooms prior to commencement of the layup operation. A track or conveyor extends from the copper/plate room into the B-stage room through an opening in a common wall. The height of the conveyor or track is about three feet and is sized so that a carrier plate can be transferred back and forth (i.e. shuttled) between the two rooms.
Prior to starting the layup process, the carrier plate is placed onto the shuttle conveyor in the copper/plate room. Kraft paper is placed onto the carrier plate to aid in pressure distribution and heat transfer adjustment, as aforesaid. After the carrier plate and kraft paper are in position, the layup process is as follow:
Steps two through seven are repeated until the book is completed. Kraft paper is placed on the last press plate, and the book is taken to a load rack or elevator to await lamination under heat and pressure.
With the foregoing shuttle system, in comparison with the standard system, there is provided better separation of the B-stage from the copper foil and press plate, theoretically reducing the probability of contaminating the plates and copper surfaces with B-stage dust particles. However, the transfer of the carrier tray through the wall connecting the two rooms does allow a certain amount of B-stage contamination to occur. Also, the above-described shuttle layup operation is about 30% slower than the aforementioned standard layup operation.
It is accordingly one objection of the present invention to minimize, during layup, particulate contamination of copper foils and press plate surfaces from resin particles and glass fiber residues.
Another object of the invention is to minimize the aforesaid particulate contamination of copper foils and press plate surfaces in a high speed continuous production process for the layup of one or more laminate books and the breakdown of such books, after curing, in an automated conveyorized system.
Another object is to positively control the flow of air within the layup stations to prevent airborne resin and glass fiber particles from traveling to the copper foil/press plate rooms from a pre-preg layup room.
Still another object is to achieve layup and breakdown of laminate books and minimize contamination of copper foil and press plate surfaces through the avoidance of overhead handling equipment and by means of minimal contact between conveyorized equipment and clean surfaces of the press plates during their conveyance in the layup process.